1 Field of the Invention
The present invention relates to a light source unit and an optical head equipped therewith, and more specifically, the present invention is directed to the improvement of a light source unit built in with a diffraction element and an optical head equipped with the light source unit.
2 Description of the Related Art
In the art of optical heads for use in optical disc drivers such as compact disc players, etc., there has been developed a technology which could reduce the number of parts in the device by employing a diffraction element (hologram optical element). Moreover, application of this technology to rewritable type optical disc systems for magneto-optical disc has been investigated in order to make the device small and light-weighted and reduce the cost of the device.
As a prior art example of such an optical head employing a diffraction element in optical system, there has been known an arrangement as shown in FIG. 1 disclosed in an article in "Sharp Technical Journal (vol.42 pp.45-52, 1989)". In FIG. 1, light emitted from a semiconductor laser 1 is diffracted by a diffraction element 2. The zero order diffracted light passes through a collimator lens 3 and the transmitted light is condensed on a recording medium 5 through an objective lens 4. The thus condensed light is reflected on the recording medium 5 and the reflected light therefrom is transmitted through the objective lens 4 and the collimator lens 3, and then diffracted by the diffraction element 2. The first order diffracted light is lead to a photodetector 6 in order to provide a focusing error signal and tracking error signal.
Now referring to FIGS. 2 and 3, the principle of detection of a focusing error signal in the thus constructed optical head will be explained. As shown in FIG. 2, when viewed from a side of the recording medium 5, the diffraction element 2 is constructed of two parts 2a, 2b divided by a parting line 2e. The divided parts 2a, 2b respectively have gratings 2c, 2c and 2d, 2d formed at right angles with the parting line 2e, with different grid pitches from one another. Here the parting line 2e is aligned in the direction of tracks on the recording medium 5. On the other hand, a photodetector 6, as shown in FIG. 3, is divided into three light receiving areas 6a to 6c. In this arrangement, when light beam emitted from the semiconductor laser 1 is in a focalized state on the recording medium 5, as shown in FIG. 3(b), the beam diffracted in the divided part 2a of the diffraction element 2 is condensed to form a spot-like beam image Q.sub.1 on a parting line 6d defined between the light receiving areas 6a, 6b in the photodetector 6, whereas the beam diffracted in the divided part 2b is condensed to form a spot-like beam image Q.sub.2 on the light receiving area 6c. When the recording medium 5 becomes near to the objective lens 4, diffracted light is focalized at behind the photodetector 6, so that unfocused semi-circular beam images Q.sub.1, Q.sub.2 are formed on the photodetector 6 as shown in FIG. 3(a). In contrast, when the recording medium 5 becomes far from the objective lens 4, diffracted light is focalized in front of the photodetector 6, so that unfocused beam images Q.sub.1, Q.sub.2 having semi-circular shapes directed opposite to the above ones are formed on the photodetector 6 as shown in FIG. 3(c). Accordingly, naming output signals obtained from light receiving areas 6 a 6c as Sa to Sc, the focusing error signal (FES) can be obtained by calculating FES=Sa -Sb.
Next, a principle of detection of a tracking error signal will be described with reference to FIGS. 4 and 5. Detection of a tracking error signal is performed using push-pull method. As shown in FIG. 4(b), when a light beam B is irradiated onto the center of a track 7 formed on the recording medium 5 as a guide groove, the intensity distribution of the returning light from the recording medium 5 is symmetric as shown in FIG. 5(b) with respect to a center line 1.sub.2 --1.sub.2 corresponding to a center line 1.sub.1 --1.sub.1 of the above-mentioned light beam B. Here, it should be noted that area in which intensity is low is hatched in FIGS. 5(a) to 5(c). On the other hand, as shown in FIG. 4(a) or 4(c), when the light beam B is irradiated on a position off the center of the track 7 inward or outward, the intensity distribution of the returning light is asymmetrical with respect to the above-mentioned center line 1.sub.2 --1.sub.2 as shown in FIG. 5(a) or (c). By utilizing such effect, alignment of the parting line 2e of the diffraction element 2 with the tracking direction can be carried out and a tracking error signal can be detected by comparing the light intensity of returning light, reflected from the recording medium 5 and diffracted in the divided part 2a, with the light intensity of returning light diffracted in the divided part 2b. Accordingly, naming output signals obtained from light receiving areas 6a to 6c as Sa to Sc, the tracking error signal (TES) can be obtained by calculating TES=Sa+Sb-Sc.
In order to effect exact recording and reproducing of information, the objective lens 4 is driven in both focusing and tracking directions by means of an unillustrated actuator based upon the focusing error signal and tracking error signal, so that the irradiated light beam may be focalized on the recording medium 5 and the condensed light beam can be positioned precisely on the track 7.
As stated heretofore, in the prior art device, every diffraction element used to be adjusted on its corresponding individual optical head observing the focusing error signal and tracking error signal, but this adjustment process has posed problems that a space should be secured in the optical head for adjusting the diffraction element; mechanical parts for the adjustment are needed; adjusting jig must be complicated; and the diffraction element is hard to secure and adhere. To solve the problems, the following procedures are taken. That is, a light source unit 8 comprising semiconductor laser 1, photodetector 6 and diffraction element 2 is assembled while the diffraction element 2 is adjusted on a substitute optical head arrangement as shown in FIG. 8, which is equipped with the equivalent optical parts as in the optical head. Then, the thus assembled unit 8 is incorporated into an actual optical head.
As the semiconductor laser 1 in the aforementioned optical head, one shown in FIG. 6 is employed generally for a wide commercial use. The semiconductor laser 1 is constructed such that laser chips 1a, 1b are joined together at a junction interface 1c, and therefore light divergent angles differ between Y-axis direction on an XY-plane parallel to the Junction interface 1c and Z-axis direction on an XZ-plane perpendicular to the junction interface 1c. In other words, intensity of light beam emitted from the semiconductor laser has a Gaussian distribution, but extent of divergent angle differs depending upon directions. Specifically, a GaAlAs type semiconductor laser with a wavelength of 780 nm, which is mainly used currently, has full angles at half height (or an angle defined by two points at which intensity of light is half as much as the central intensity) of about 10.degree. in Y-axis direction and about 30.degree. in Z-axis direction. Accordingly, a pattern in a YZ-plane cross section which is perpendicular to the optical axis of the light beam has an elliptic shape having its minor axis on Y-axis and its major axis on Z-axis. The laser chips 1a, 1b and the photodetector 6 are integrally housed in a package 10 as shown in FIG. 7. The package 10 containing the laser chips 1a, 1b and the photodetector 6 is hermetically sealed with a glass window 10a so as to protect the inside from the outside air containing humidity, oxygen, etc. The diffraction element 2 is secured adhesively on the outer side of the glass window 10a to complete the light source unit 8. A positional displacement and/or deflection of angle occur when the laser chips 1a, 1b are built in.
Accordingly, in the adjustment of an optical head by using the substitute optical head arrangement shown in FIG. 8, the package including the semiconductor laser 1, collimator lens 3' and objective lens 4' are centered with mechanical precision, but the positional displacement and/or deflection of angle in fitting the laser chips 1a, 1b may occur, so that emission of the laser beam may be displaced. The central optical path in intensity distribution of the emitted laser beam is shown with a solid line. The emitted light beam from the semiconductor laser 1 is converted into a parallel beam through the collimator lens 3'. The propagating direction of the collimated light is alined with a line (shown with a single chain line in FIG. 8) joined between the point from which the laser beam is emitted and the central point of the collimator lens 3'. The objective lens 4' focalizes the light coming from the collimator lens 3' on a point on a line (shown with a broken line in FIG. 8) which is parallel to the collimated beam from the collimator lens 3' and passes through the center of the objective lens 4'. The light incident on the recording medium 5 is reflected in such a direction that suffices the relation that incident angle be equal to reflection angle. The reflected light then enters the objective lens 4' once more. The reflected light having passed through the objective lens 4' proceeds in a direction parallel with the incident light but opposite under the effect of the lens 4'. Then, the light is condensed by the collimator lens 3' to proceed toward the position of the semiconductor laser 1. As shown in FIG. 8, the center of intensity distribution on the diffraction element 2 for the returning light is largely deviated from that for the outgoing light. The position of the diffraction element 2 is adjusted based on the intensity distribution of the returning light so as to obtain predetermined focusing error signal and tracking error signal.
In an actual optical head, the center of intensity distribution for the incident light onto the objective lens is adjusted such that it coincides with the optical axis of the objective lens 4 in order to improve the spot performance, in order to improve the spot performance, and the incident light on the recording medium 5 be incident perpendicularly thereon. Specifically, as shown in FIG. 9, the semiconductor laser 1 is adjusted and positioned along a direction perpendicular to an optical axis with respect to the collimator lens 3. That is, the optical axis is adjusted so that the propagating beam from the collimator lens 3 may be made incident perpendicularly onto the recording medium. Thereafter, the intensity distribution is adjusted by shifting the integrated luminous block 9 made of both the semiconductor laser 1 and collimator lens 3 in a direction perpendicular to the optical axis. Since the adjustment described above are effected, in assembling the light source unit 8 adjusted on the substitute optical head arrangement into an actual optical head, position of the semiconductor laser 1 is adjusted with respect to the collimator lens 3 so that the parallelized beam outgoing from the collimator lens 3 may be made incident perpendicularly on the recording medium 5 while the integrated luminous block 9 of the semiconductor laser 1 and collimator lens 3 is positionally adjusted with respect to the objective lens 4 so the center of the objective lens be coincident with that of the intensity distribution of the beam. Accordingly, the center of intensity distribution of the outgoing light beam coincides with that of the returning light beam. Nevertheless, since the diffraction element 2 is adjusted with reference to a center of the returning path which is deviated from that of the outgoing path under the influence of the deflection or displacement of emitted light direction due to the fitting position of the laser chips 1a, 1b and/or dispersion of the attached orientation, the error signals observed on the actual optical head differs from those observed on the substitute optical head arrangement. Accordingly, offset of the focusing error signal and tracking error signal may or does occur.
The degree of the offset depends upon the deflection of the emitted light orientation, and is influenced by the numerical aperture of the collimator lens 3, or the utilizing range of the intensity distribution of the emitted laser beam. For example, the numerical aperture of a collimator lens 3 used in an optical head for compact disc is as small as 0.1, so this allows use of only central portion, which can be assumed uniform, of the emitted laser beam having anisotropic intensity distribution, so that the displacement of the intensity distribution described above may be less influential. In contrast, for instance, an optical head for use in that rewritable type optical disc system for magneto-optical disc which is capable not only of reproducing signals but also recording information, need be increased in light utilization in order to obtain light intensity enough to perform recording. For this reason the numerical aperture of the collimator lens is set to be as large as 0.3 to 0.4, therefore, almost full range of the intensity distribution of the emitted laser beam must be utilized. This makes large the influence of the displacement of the intensity distribution.